Formation of durable lubricious coatings have been found to be of great importance in the glass container industry to provide glass articles, or ware, with at least one layer of an adhering lubricating material in order to facilitate high speed automatic handling of glass articles in production lines and to protect articles against contact abrasion damage and unsightly scuff and scratch marks. Uncoated glass articles are highly susceptible to abrasion damage, and it has been reported that newly formed uncoated glass articles can quickly lose up to 75% of their bursting strength due, at least in part, to surface abrasion caused by contact with other glass articles, as normally occurs during processing and handling of such articles.
While some coatings have been applied to articles just prior to use, to be fully effective, the articles must be coated soon after they are formed, and in the case of annealed articles, for example, such coatings have been applied immediately before and/or after annealing.
In practice, pre-annealing coatings, sometimes referred to as "hot end" coatings, are applied to glassware after it leaves the glassware machine in an initial coater. The initial coater forms a very thin metal oxide coating on the outer surface on the surface of the glassware, which is then carried to the annealing lehr. Commonly used metal oxides include tin compounds and titanium oxide. Such pre-annealing coating methods and apparatus are disclosed, for example, in U.S. Pat. Nos. 4,431,692; 4,615,916; 4,668,268; 4,719,126; and 4,719,127 and others listed below.
A number of post-annealing coatings, sometimes referred to as "cold end" coatings, and methods and apparatus for their application, have been disclosed, for example, in U.S. Pat. Nos. 2,995,633; 3,386,855; 3,487,035; 3,712,829; 3,801,361; 3,876,410; 3,989,004; 3,997,693; 4,039,310; 4,130,407; 4,135,014; 4,517,242; 4,517,243; 4,529,657; and 4,812,332.
Electrostatic deposition methods and apparatus are well known. Such methods and apparatus have been in common use in industry to apply various useful, protective and decorative coatings. Examples of such electrostatic coating methods and apparatus include U.S. Pat. Nos. 2,685,536; 2,794,417; 2,893,893; 2,893,894; Re. 24,602; 3,048,498; 3,169,882; 3,169,883; 3,323,934; 3,991,710; 4,073,966; 4,170,193 and many others. Notwithstanding their extensive development and use, electrostatic coating methods and apparatus have not been used in the application of cold end coating materials to glass containers.
U.S. Pat. Nos. 3,876,410 and 3,989,004 disclose the use of a coating material that is, at least in part, vaporizable at a readily obtainable temperature and capable of producing vapor that is contact-adherent to the article to be coated to produce a durable and tenacious, lubricious coating. In general, the patents disclose that an acceptable coating material can be formed from organic materials, particularly hydrocarbons formed from methylene, ethylene, propylene, butylene, fatty acids and their derivatives and the like, and that to be particularly effective, the vapor molecules of the coating material should be of a polar-non-polar nature such that the polar portion of the molecule will tend to adhere strongly to the article to be coated and oriented so that the non-polar portion of the molecule forms the lubricious external surface. A particularly useful group of such coating compositions disclosed in these patents are the saturated and unsaturated fatty acids containing between 10 and 18 carbon atoms. When used in the method of these patents, such coating materials are vaporized and conducted to the vicinity of newly formed glassware and readily adhere to the glassware in a thin, clear, tenacious, lubricious coating. To maintain the surface energy of the glassware at a high level, the glassware is maintained at a temperature between about 100.degree. F. ( 37.degree. C.) and 325.degree. F. (162.degree. C.) and preferably at a temperature between about 120.degree. F. (49.degree. C.) and 250.degree. F. (121.degree. C.).
Caporic acid, stearic acid, oleic acid, myristic acid, linolic acid and palmatoleic acid [sic]are disclosed as typical of the compositions yielding desirable coatings on glassware when used according to the method of the above patents.
Of the compositions disclosed in these patents, the preferred composition is oleic acid. Oleic acid is a bland liquid in normal condition having an appearance similar to that of cooking oil. It has been approved for use in connection with food products. As little as 1 drop of oleic acid every 17 seconds has been found sufficient to produce a superior lubricating coating on catsup bottles passing through the vapor at the rate of 80 bottles per minute Thus, 1 drop of oleic acid provides sufficient vapor to coat about 20 catsup bottles. Further, oleic acid is readily available in high-grade quality at low cost. Because of these advantages, oleic acid has been a primary coating material used to provide post-annealing (cold end) coatings.
Oleic acid, however, is liquid at temperatures in excess of 57.degree. F. (14.degree. C.). When glassware coated with oleic acid is exposed to elevated temperatures, such as in an autoclave for the sterilization of food containers, the oleic acid coating is substantially removed by the harsh and hot conditions, and the scratch resistance and lubricity are deleteriously affected, thereby increasing the risk of breakage. Breakage during processing is serious because of the possibility of slivers or fragments of the shattered glass being deposited in adjacent ware, which is undesirable in most situations and is completely unacceptable when the ware is to be used for food packaging.
It is also important that any coating material applied to glassware used with food or liquid products does not leave a residual taste of any sort in the food or liquid. Indeed, oleic acid is commonly known to impart a bad taste to the contents of the glassware, and is also known to adversely affect the foaming of carbonated beverages.
Among the above-identified patents, U.S. Pat. Nos. 4,039,310 and 4,130,407 disclose methods of strengthening glass against failure. U.S. Pat. No. 4,039,310 discloses a method of strengthening glass by heating the glass to a temperature in excess of 700.degree. F. (371.degree. C.) but below the decomposition temperature of a selected fatty acid, such as behenic, stearic or glutamoric acid and applying the fatty acid to the heated glass. U.S. Pat. No. 4,130,407 discloses a method of strengthening glass by applying a fatty acid derivative of an inorganic salt at temperatures between 100.degree. C. (212.degree. F.) and 500.degree. C. (932.degree. F.).
With conventional coating methods, coating material is often needlessly applied to the interior of the glassware. Existing methods and apparatus for coating glassware do not preclude the deposition of coating material to the interior of the glassware. In many applications of glassware this presents no problem, even with such cold end coatings as oleic acid. However, a serious problem exists with respect to glass bottles used to contain beer, carbonated soft drinks and other fragile food stuffs whose taste, foaming or other characteristics may be damaged by glassware coating materials. Not only can an interior coating often taint the taste of beverages stored in the bottle, but such coatings, if applied to the exterior of the mouth of the bottle, can frequently leave a residual taste in the mouth of a person drinking directly from the bottle, which is common in the use of beer or soft drink bottles.
A need exists in the manufacture of coated glass containers to coat the containers to provide an inexpensive, thin, tenacious and protective coating without affecting important desirable attributes of the containers' contents, such as taste, foaming and the like.